The present invention relates to an electronic flash device and, more particularly, to an electronic flash device which can be suitably built in or externally attached to various cameras.
Conventionally, a camera which is used to take a sliver halide photo or a digital camera that senses an image using an image sensing element prevalently comprises a so-called electronic flash device that instantaneously illuminates an object upon photographing.
The conventional electronic flash device typically comprises a charge circuit for boosting a power supply voltage, and charging light emission energy for a discharge tube, and a trigger circuit for triggering (initiating) light emission. The charge circuit and trigger circuit respectively adopt coil transformers, as disclosed in, e.g., Japanese Patent Laid-Open No. 2000-241860.
However, since the coil transformer normally has a large outer shape and is heavy in weight, it is not suitable for an electronic flash device for a camera, which is carried by the user.
The present applicants have proposed an electronic flash device in which a charge circuit and trigger circuit respectively adopt piezoelectric transformers, which are lightweight and compact compared to the coil transformer, and have a high degree of freedom in element shape, as shown in FIG. 7, in Japanese Patent Application No. 2000-311384 (not laid-open at the time of filing of this application) filed previously.
That is, the electronic flash device shown in FIG. 7 roughly comprises a charge circuit 21, trigger circuit 22, discharge capacitor 23, and discharge tube 24.
In this circuit, when a switch 52 is ON(CLOSE) and a switch 58 is OFF(OPEN), a driving circuit 53 drives a piezoelectric transformer 54 in accordance with an oscillation signal output from an oscillation circuit 51. An output voltage from the piezoelectric transformer 54 is rectified by a rectifier circuit formed by diodes 55 and 56, and then charges the discharge capacitor 23 as electric energy that makes the discharge tube 24 emit light. When the switch 52 is OFF and the switch 58 is ON, a driving circuit 59 drives a piezoelectric transformer 60 in accordance with an oscillation signal output from an oscillation circuit 57, and the discharge tube 24 emits flash light by energy stored in the discharge capacitor 23 in response to a high voltage output from that piezoelectric transformer as a trigger.
However, in order to attain a further size reduction of the electronic flash device, it is not sufficient to adopt the piezoelectric transformers in the charge circuit 21 and trigger circuit 23, as shown in FIG. 7.
The present invention has been proposed to solve the conventional problems, and has as its object to provide an electronic flash device which has a simple circuit arrangement and saves space.
In order to achieve the above object, an electronic flash device according to the present invention is characterized by the following arrangement.
That is, an electronic flash device which comprises a discharge tube (4) for emitting flash light, a capacitor (3) for storing electric energy that makes the discharge tube emit light, a charge circuit (1) for charging the capacitor with electric energy, and a trigger circuit (2) for generating a high-voltage signal that triggers the discharge tube to discharge, further comprises a booster circuit (11, 11A, 12, 13) which includes a single piezoelectric transformer and is commonly used as booster means of the charge circuit and the trigger circuit, and switching means (14, 14A-14D) for switching the booster circuit for one of the charge circuit and the trigger circuit so as to apply an output voltage of the piezoelectric transformer to one of the capacitor and the discharge tube.
In a preferred embodiment, the switching means comprises at least one switch or switching element (14) which is provided in series in a line that connects an output of the piezoelectric transformer and the capacitor (see FIG. 1).
In another preferred embodiment, the switching means comprises at least one switch or switching element (14A-14D) for switching an output of the piezoelectric transformer to one of a first line that connects the output to the capacitor and a second line that connects the output to a trigger line of the discharge tube (see FIGS. 2 and 3). In this case; a high-impedance load (17) is preferably connected in parallel with an output of the piezoelectric transformer to prevent the output of the piezoelectric transformer from being fully opened upon switching the switch or switching element.
In either arrangement, an oscillation circuit (11A) of the piezoelectric transformer in the charge circuit and the trigger circuit can output a plurality of types of oscillation signals having different frequencies, and preferably outputs one of these oscillation signals in response to a switching operation of the switching means (see FIG. 4). More specifically, the frequency of the oscillation signal is preferably switched from a low frequency for charge to a high frequency for trigger, as shown in FIG. 5C.
Or the device further comprises a charging voltage detection circuit (18) for detecting a charging voltage of the capacitor, and, in the charge circuit and the trigger circuit, an oscillation circuit (11A) of the piezoelectric transformer can output a plurality of types of oscillation signals having different frequencies, and preferably outputs one of these oscillation signals in response to an output signal from the charging voltage detection circuit (see FIG. 6). More specifically, the frequency of the oscillation signal is preferably switched from a low frequency for charge to a high frequency for trigger, as shown in FIG. 5C.
Alternatively, in order to achieve the above object, an electronic flash device which comprises another device arrangement according to the present invention is characterized by the following arrangement.
That is, an electronic flash device which comprises a discharge tube for emitting flash light, a capacitor for storing electric energy that makes the discharge tube emit light, a charge circuit for charging the capacitor with electric energy, and a trigger circuit for generating a high-voltage signal that triggers the discharge tube to discharge, further comprises a booster circuit (11, 12, 13A) including a single piezoelectric transformer (13A), which is commonly used as booster means of the charge circuit (1) and the trigger circuit (2), and, in that the booster circuit, two secondary electrodes (6, 7) are provided to a secondary region of the piezoelectric transformer at two positions of different distances (L1, L2) from a primary electrode (5) provided to a primary region, one of the secondary electrode serves as a part of the booster means of the charge circuit and the other serves as a part of the booster means of the trigger circuit. Preferably, the secondary electrode (6) closer to the primary electrode of the two secondary electrodes of the piezoelectric transformer serves as a part of the booster means of the charge circuit, and the secondary electrode (7) farther from the primary electrode serves as a part of the booster means of the trigger circuit.
With this device arrangement, since a piezoelectric transformer provided to a single booster circuit uses its two secondary electrodes respectively for a charge circuit and trigger circuit, the charge and trigger functions can be achieved at the same time by the single booster circuit. That is, a compact electronic flash device can be realized by a simple device arrangement.
Note that the electronic flash device with any of the above arrangements can be suitably built in or externally attached to various cameras. In this case, the trigger circuit preferably operates in response to shutter operation of the camera.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.